This invention relates to a method of removing trace elements from aqueous solutions and more particularly to the treatment of effluents and purge streams containing dissolved mercury and other contaminating elements.
One major problem in the chemical industry is the removal of an increasingly large number of pollutants from waste water and other process streams. Following the lead of the Federal Water Pollution Control Act of 1972 (commonly referred to as the "Clean Water Act"), many states, counties, and even cities are posting ever more stringent regulations concerning the discharge of a wide variety of polluting materials, both organic and inorganic. One group of materials causing particular concern is the "priority" pollutants as established under Section 307 of the Clean Water Act. These have been determined to present unusual hazards in terms of toxicity, carcinogenicity and/or mutagenicity. Presently included in this list are ions and salts of some 13 elements which, along with the current federal drinking water and maximum allowable river discharge limits, are given in Table I.
TABLE I ______________________________________ Maximum Allowable EPA Priority Concentrations Pollutant Elements (ppm) ______________________________________ Antimony 0.15 Arsenic 0.05 Beryllium 0.000037 Cadmium 0.01 Chromium 0.05 Copper 1.0 Lead 0.05 Mercury 0.002 Nickel 0.013 Selenium 0.01 Silver 0.05 Thallium 0.013 Zinc 5.0 ______________________________________
In treating aqueous streams to remove these elements, it is found that these elements are present in various forms including cationic, anionic, non-ionic elements or complexes, or bound in suspended particulates. Consequently, a plurality of treatments may be necessary to more or less completely remove them. This imposes a significant capital and operating cost penalty on any facility which is required to handle complex process chemicals in waste water streams, especially those having several of these pollutants present at the same time.
This situation arises, for example, in the chlor-alkali industry where anolyte brines removed from membrane or mercury cells nominally contain amounts of metal contaminants such as cadmium, chromium, copper, lead, nickel, and zinc which exceed the EPA maximum allowable discharge limits. In addition, these brines contain contaminating amounts of metals such as titanium and iron while mercury cell effluent brines contain mercury concentrations in the range of 1 to 20 parts per million (ppm). The brines also contain substantial amounts of non-pollutant sulfate, chlorate and alkaline earth metal ions such as calcium and magnesium as well, all in a concentrated brine of the alkali metal being electrolyzed. Where, in the past, part of the brine was periodically purged by discharging into waterways, to provide, for example chlorate and sulfate control, this is no longer possible in many locations.
This problem has been recognized for quite some time and a variety of approaches have been developed to solve it. Many of these involve treating either the dechlorinated brine or the purge stream with one or more chemicals either to reduce the dissolved mercury and other heavy metals to the metallic form or convert them to insoluble products, such as sulfides, which can be filtered out. Such treatments are often quite vigorous since, for example, mercury in concentrated brine solution is known to form complexes which are very stable. In still other cases, the brine is treated with hydrocarbons or other solvents to extract these complexes, but whichever of these methods is used, they all share the common problems of added costs and complexity in what is basically a low cost/high volume process.
One approach to pollution abatement which appears to offer unusual cost effectiveness is the use of one or more chelating agents as absorption compounds. Capable of operating in a wide variety of chemical environments, such materials are finding wide use in treating process and waste water streams to selectively remove a considerable number of heavy metal compounds. However, most, if not all, of the commercially available agents used for this purpose are quite expensive and are sensitive to dissolved calcium and magnesium. Thus, for many applications, their real utility in comparison to alternative methods for treating purge streams is quite limited.
One group of chelating agents showing unusual promise are poly(dithiocarbamate) resins such as those prepared by Dingman et al, Hackett and Siggia, and Miyazaki and Barnes.
Dingman et al prepared poly(dithiocarbamate) resins having a sulfur content of 9.5% by reacting equal amounts by weight of polyethyleneimine with toluene diisocyanate, using dioxane as the solvent. The reaction product was further treated with carbon disulfide and allowed to react for three weeks (Analytical Chemistry 46, No. 6, pages 774-777, May, 1974). The poly(dithiocarbamate) resins were found to be able to chelate a heavy metal ion such as silver, mercury, copper, lead and nickel from its aqueous solution.
Hackett and Siggia reported the preparation of poly(dithiocarbamate) resins in "Selective Concentration and Determination of Trace Metals Using Poly(dithiocarbamate) Chelating Ion-Exchange Resins" in Environmental Analysis, edited by G. W. Ewing, Academic Press, Inc., New York, New York 1977, pages 253-265.
Their procedure for making these resins comprised reacting, in dioxane solution, an 8:1 mixture of an anhydrous polyethyleneimine-1800 molecular weight and a polymethylene polyphenylisocyanate to form a solid polyamine-polyurea crosslinked precursor. This, in turn, was reacted with a mixture of NH.sub.4 OH and CS.sub.2 in isopropyl alcohol over a period of about 4 weeks to form resins having a sulfur content of about 18 percent and an equivalent Cu.sup.+2 capacity (milliequivalents of Cu absorbed/gram of dry resin from an aqueous solution at a pH of about 4.8) of between about 0.8 and 1.35. Hackett and Siggia removed heavy metals such as these studied by Dingman et al from solutions including sea water and milk.
Miyazaki and Barnes have reported (Analytical Chemistry, 53, No. 2, pages 299-304, Feb., 1981) that the NH OH/CS.sub.2 reaction time in the method of Hackett and Siggia can be reduced to as short a time as 8-16 hours. The poly(dithiocarbamate) resin prepared had a sulfur content of about 18 percent and was used in the chelation of rare earth elements and metals including chromium, titanium, vanadium, molybdenum, tungsten, and osmium from aqueous solutions.
The common elements in these prior art studies are the use of dioxane as the solvent; anhydrous polyethyleneimine used as a precursor reactant and the belief that a high sulfur content resin was required to effectively remove metal contaminants. Dioxane is a federally listed health hazard and it would be highly desirable if less hazardous solvents could be used. It is known that polyethyleneimine can be supplied as an aqueous suspension at attractive prices. However, it is, at best, only sparingly soluble in dioxane and attempts to form a satisfactory precursor resin from such a material almost invariably end in failure. The reason for this appears to be that the water in the suspension saturates the dioxane thus effectively inhibiting its ability to dissolve the polyimine so that very little, if any, is available to react with the polyisocyanate. In addition, the processes of the prior art for preparing poly(dithiocarbamate) resins employ high ratios of polyethyleneimine to polyisocyanate which result in increased raw material costs. Further, the poly(dithiocarbamate) resins containing chelated priority pollutants and contaminants must be properly disposed of in a way which minimizes both disposal costs and environmental risks.